Single instance base class for C++.

main.cpp

#include <iostream>
#include "single_instance.hpp"

using namespace std;

class A : public enable_single_instance<A>
{
    friend class enable_single_instance<A>;
    
public:
    void echo()
    {
        cout << this << endl;
    }

    ~A() 
    {         
        cout << __FUNCTION__ << endl;
    }

private:
    A() 
    { 
        cout << __FUNCTION__ << endl;
    }
};

int main(int argc, char* argv[])
{
    A::single_instance().echo();
    A::single_instance().echo();
    A::single_instance().echo();

    return 0;
}

single_instance.hpp

#pragma once

#include <memory>
#include <mutex>

template <typename T>
class enable_single_instance
{
    friend T;

public:
    static T& single_instance()
    {
        std::call_once(_flag, [&]{ _instance.reset(new T()); });
        return *_instance;
    }

private:
    enable_single_instance() {};

private:
    static std::unique_ptr<T> _instance;
    static std::once_flag     _flag;
};

template <typename T> std::unique_ptr<T> enable_single_instance<T>::_instance;
template <typename T> std::once_flag     enable_single_instance<T>::_flag;